In an engine fuel system having a plurality of fuel injectors, it is typically desirable that each injector deliver approximately the same quantity of fuel in approximately the same timed relationship to the engine for proper operation. Several problems arise when the performance, or, more particularly, the timing (i.e., the time between the application of a fuel delivery command and the Start of Injection (SOI)) and delivery (i.e., the quantity and pressure of the delivered fuel) of the injectors diverge beyond acceptable limits. One problem caused by injector performance deviation or variability is that different torques are generated between cylinders due to unequal fuel amounts being injected, or from the relative timing of such fuel injection. Further, knowledge that such variations are inevitable require engine system designers to account for this variability; accordingly, many engine systems are designed not for peak or maximum cylinder pressures or output, but rather, are designed to provide an output equal to the maximum theoretical output less an amount due to the worst case fuel injector variability.
One approach for solving these problems in unit injectors is the so-called select fit manufacturing process. Generally, a common procedure involves flowing fluid through each unit injector nozzle and pumping mechanism and categorizing each nozzle and pumping mechanism accordingly. During assembly, nozzles are matched with pumping mechanisms knee to be compatible, depending on the category into which each was categorized. The disadvantage associated with this approach is the relatively high cost involved with sorting the nozzles and pumping mechanisms and maintaining these groupings for the duration of the manufacturing and assembly process,
Another approach for solving these problems involves extremely rigid manufacturing procedures for achieving high manufacturing precision necessary to meet the desired design specification. Such high manufacturing precision has the disadvantage of increasing the manufacturing cost, including the costs involved in manufacturing precision components and subassemblies and the costs related to the subsequent assembly process. Further, neither of the above-mentioned manufacturing-oriented solutions satisfactorily controls rejection of completely assembled injectors that fail to fall within the timing and delivery tolerances of the design specification. Thus, excess scrap remains a problem with these manufacturing-oriented approaches.
With the advent of increasingly sophisticated electronic control, a new approach to the problem of timing and delivery variations has emerged. In known electronic fuel injection systems, especially diesel-cycle internal combustion engine systems, the timing or start of injection, as well as the end of injection, or duration (delivery) is controlled by an electronic control, which controls these parameters for all of the engine cylinders.
An early attempt at using an electronic control to compensate for individual injector timing and delivery variations in a engine system involved measuring the flow characteristics of a particular injector at a single operating condition, and obtaining constants from this empirical testing, relative to an ideal fuel injector, and using these constants to modify a nominal control signal to compensate for the measured variation. This approach has proven unsatisfactory because it does not take into account the fact that timing and delivery variations exist not only between injectors, but as a function of the particular operating condition at which the injectors are operated. For example, it may be observed that at a low speed, low load condition, an individual injector may have greater variability from nominal specifications than at a high speed, high load condition. Thus, this approach has failed to provide a reduced injector to injector and injector to nominal performance variation necessary to meet today's increasingly strict emission standards.
Others have tried to compensate for variation in the start of injection characteristic of individual injectors in an engine system by designating a proxy for the timing or the start of injection characteristic of the injector. In general, these methods first electrically detect the closure of a valve used in controlling the start and duration of fuel injection, in response to an injection command. These methods further assume that the time between valve closure and the start of injection is fixed. Given these two time intervals, the injection command can be modified to compensate for variation in the time between the injection command and valve closure. The problem that remains with this type of approach is that the detected valve closure does not precede the start of injection by a fixed time period. Many factors, including manufacturing and assembly variations, contribute to vary the actual start of injection from a nominal value. Thus, this approach does not eliminate injector to injector and injector to nominal variation due to variations of the valve-closure to start of injection time interval.
Accordingly, there is a need to provide an improved method and structure for controlling an apparatus, such as a fuel injector, that minimizes or eliminates one or more of the problems as set forth above.